As the label for control of articles such as labels attached to plastic containers used for transporting foods (returnable containers), labels used for control of electronic parts and labels attached to cardboard boxes for control of distribution of articles, currently, labels having a heat-sensitive recording material are mainly used.
In the heat sensitive recording material, a heat-sensitive recording layer containing an electron-donating dye precursor which is, in general, colorless or colored slightly and an electron-accepting color developing agent as the main components is formed on a support. Rewritable labels which allows formation of an image, erasure of the formed image and rewriting of another image are increasingly used recently. When it is desired that the label attached to an adherend is treated for rewriting without detaching the label from the adherend, the erasure of the recorded image and the rewriting of another image cannot be conducted by passing through an ordinary printer since the label remains attached to the adherend. To conduct the desired treatment, it is necessary that the erasure and the rewriting of images be conducted without contacting the label.
For the repeated use of a label, in recent years, reversible heat sensitive recording materials which allows formation and erasure of images, such as (I) a reversible heat sensitive recording material having a heat sensitive layer which is formed on a substrate and contains a resin and an organic low molecular weight substance showing reversible changes in transparency depending on the temperature and (II) a reversible heat sensitive recording material having a heat sensitive color developing layer which is formed on a substrate and contains a dye precursor and a reversible color developing agent, have been developed.
Among the above reversible heat sensitive recording materials, reversible heat sensitive recording material (II) is more widely used. However, the heat sensitive color developing layer in the above heat sensitive recording material has a drawback in that the surface of the label is destroyed or the developed color is lost to decrease the density of recording and the visibility when the surface is subject to a specific heat history, specifically, when the surface irradiated with laser light is irradiated again or a plurality of line elements are recorded while the surface is at a temperature in a specific range. Due to this drawback, a problem arises in that, when a one-dimensional bar code or a solid image constituted with a plurality of line elements is formed, the density of color in a potion where the color development has been made decreases due to heat generated in surrounding portions, and readability and visibility of the bar code decrease. A further problem on the formation of an image arises in that, when the time between the end of drawing the line in the previous step and the start of drawing the line in the next step is extremely short, the starting point of the line drawn in the previous step does not conform to the starting point of the line drawn in the next step, and a clear image is not formed. Due to this problem, visibility becomes insufficient, and readability of the bar code decreases.
[Patent Reference 1] Japanese Patent Application Laid-Open No. 2003-118238
[Patent Reference 2] Japanese Patent Application Laid-Open No. 2003-320694